1. Field of the Invention
The present invention relates to a method for separating water-miscible organic species from a process stream by aqueous biphasic extraction. In particular, the method includes extracting the organic species into a polymer-rich phase of an aqueous biphase system in which the process stream comprises the salt-rich phase, and, next, separating the polymer from the extracted organic species by contacting the loaded, polymer-rich phase with a water-immiscible organic phase. Alternatively, the polymer can be separated from the extracted organic species by raising the temperature of the loaded, polymer-rich phase above the cloud point, such that the polymer and the water-soluble organic species separate into two distinct aqueous phases. In either case, a substantially salt-free, concentrated aqueous solution containing the organic species is recovered.
2. Description of Prior Art
Aqueous biphasic extraction involves the selective partitioning of species between two immiscible aqueous phases. These aqueous/aqueous systems form spontaneously from mixtures of aqueous solutions of unlike polymers, typically a straight-chain polymer such as polyethylene glycol (PEG) and a highly branched polymer such as dextran. Aqueous biphase systems may also be generated from mixtures of organic polymers and inorganic salt solutions. In either case, water is the major component of both phases. For large-scale applications, the polymer/salt biphase systems are generally more commercially attractive than polymer/polymer systems because of their lower cost and higher liquid/liquid phase separation rates.
Solutes present in the biphase system will selectively partition into one of the phases according to the relative solubility of the solute in the two phases. The partition behavior of a solute is described by the partition coefficient, which is the ratio of the solute concentration in the less-dense phase to the solute concentration in the more-dense phase. Phase transfer agents may be added to enhance the selective partitioning of solutes between the two phases. In this regard, aqueous biphase systems are analogous to conventional oil/water solvent extraction systems. However, unlike conventional solvent extraction systems, the two immiscible phases in an aqueous biphase system share a common diluent--water. Therefore, the potential for process upset due to excessive mass transfer of water must be carefully considered. To prevent the net mass transfer of water between the liquid phases, the polymer-rich and salt-rich phases can be contacted at compositions defined by the tie lines from the appropriate polymer/salt/water phase diagram.
Development of an efficient extraction system depends on the ability to manipulate the partitioning behavior of solutes, which can be accomplished by changing the properties of the solvents or the solute. Variables that influence the properties of the solvent include the type, molecular weight, and concentration of the phase-forming polymer, the type and concentration of phase transfer agents, the pH, and the temperature. Variables that influence the properties of solutes are the pH of the system, the type and concentration of phase transfer agents, and structural modifications, including chemical treatment of the solute.
Currently, aqueous biphase systems are in wide use as a highly efficient separation and purification technique for biological materials, including proteins, nucleic acids, peptides, cells, subcellular organelles, and viruses. For example, U.S. Pat. No. 5,308,503 to Strom teaches a method for separating microbial contaminants, including bacteria and fungi, from industrial lubricating oils and cutting liquids using an aqueous polymeric biphase system. The less-dense phase of the biphase system contains a low-molecular-weight polymer and the cutting liquid, and the more-dense phase contains a high-molecular-weight polymer or an inorganic salt, each phase having a high water content. The phases are mixed together and transferred to a separator, wherein the cutting liquid concentrates in the less-dense polymer phase and the microbial contaminants collect in the more-dense phase.
Recently, aqueous biphase systems have been recognized as a highly adaptable separation technique for the selective separation and recovery of particulates from solid and liquid waste streams, including the processing of mineral ores and the treatment of solid radioactive wastes, liquid nuclear wastes, and contaminated soils. A need in the art continues to exist for low cost, highly efficient, large-scale separation procedures for the extraction of inorganic and organic solutes from various industrial process streams.
A long-standing problem associated with aqueous biphase systems is the inability to efficiently recover, or back extract, the separated and concentrated solute from the polymer-rich phase. This is primarily due to the relatively high salt concentrations required to maintain the aqueous biphase system.
Solid support-based biphase separation systems, or solid/liquid systems, have been developed to overcome the back extraction problem associated with the aqueous biphase liquid/liquid system. For example, U.S. Pat. No. 5,603,834 to Rogers, et al., discloses a method for recovering pertechnetate ions from an aqueous solution by using resins impregnated with a water-soluble polymer. Additional approaches include treating the loaded polymer phase with ion exchange resins or back extracting the solutes into secondary, salt-rich aqueous phases. These processes, however, are inefficient and generate significant amounts of waste.
The present invention is a method for separating water-soluble organic species from a process stream by aqueous biphasic extraction that overcomes the back extraction difficulties experienced in the prior art. During the extraction step, a process stream comprised water, salt, and the organic species of interest is contacted with an aqueous polymer solution to generate an aqueous biphasic extraction system. The organic species partition to and concentrate in the polymer-rich phase, and the phases are separated. Next, in a polymer recovery step, the polymer-rich phase containing the extracted organic species is contacted with an organic solvent. The polymer concentrates in the organic phase, and this phase is separated from the water-soluble organic species, which are recovered in a substantially salt-free solution. The remaining organic phase containing the polymer is subjected to further method steps, resulting in recyclable, secondary streams of the phase-forming components.
In an alternative embodiment, the polymer recovery step is performed by raising the temperature of the loaded, polymer-rich phase above the cloud point, thereby causing the polymer and the organic solutes to separate into two distinct phases.
An advantage of the aqueous biphase partitioning system is that the components, which include inorganic salts, and water soluble polymers, and about 60 to 90 wt % water, are all inexpensive, nontoxic, and non-flammable. In addition, the phase-forming components of the system can be recovered for recycling, resulting in a continuous process operable on an industrial scale.
Therefore, in view of the above, a basic object of the present invention is to provide a low cost, highly efficient, and continuous method that can be readily scaled-up for separating water-miscible organics from industrial process streams.
Another object of this invention is to provide a method for recovering organic species from a process stream using aqueous biphasic extraction, including the separation of the organic species from a polymer-rich phase.
Another object of this invention is to provide a method for separating organic species from a processes stream using aqueous biphasic extraction, including the recovery and recycle of phase-forming components.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.